This invention relates to a method and apparatus for thickening mixtures of solids and liquids by the removal of some of the liquid from the mixture by means of filtration and, more particularly, relates to the backwashing of filter elements for the disengagement of filtercake therefrom.
More specifically, the invention relates to a novel method and apparatus for creating a reversal of pressure across a filter element to perform a necessary backwash in well-known pressure filters that operate cyclically (for example, U.S. Pat. Nos. 1,716,040, 3,356,215 and 4,243,533; and U.K. Pat. Nos. 603,443 and 904,946). The filtration in such apparatus is effected by a number of generally circular hollow filtering elements submerged in a vessel containing the mixture to be filtered and thickened. The vessel is supplied with the mixture by a valved inlet. The filter elements include a rigid mesh tube or perforated pipe closed at the lower end and open at the upper end thereof. The tubes are enveloped by suitable filtering media such as synthetic filter fabrics (for example, as referred to in U.S. Pat. Nos. 1,716,040, 3,233,739 and 3,356,215). The filter elements are suspended hermetically from a perforated deck within the vessel and the interior of the elements is in open contact with an overlying filtrate compartment of the vessel where the filtered liquid, i.e. as filtrate, accumulates and from where the filtrate is discharged. The bottom of the vessel contains a valved outlet for discharge of the thickened product and may also contain a rake or mixer to assist in the removal of the thickened product. The filtration cycle consists of a period of filtration followed by backwashing or cleansing of the filter elements. During the filtration period the solids, not being able to pass through the filter media, collect on the outside surfaces of the filter elements to form a layer of filtercake, while the filtered liquid flows through the elements as filtrate to accumulate in the overlying filtrate compartment from where it is withdrawn. During the backwash period the filtercake is disengaged from the surfaces of the filter elements by causing the filtrate to flow in the reverse direction for a short period of time. The filtercake pieces, being heavier than the surrounding liquid-solids mixture, settle towards the bottom of the vessel from where they are extracted, together with some of the untreated mixture trapped between the filtercake pieces, as a thickened product.
The filtration period normally is continued until the flow of filtrate is reduced to a degree that is considered unproductive due to the progressive blinding of the filter element surfaces by the build-up of filtercake. As the build-up occurs, the pressure differential between the inside and outside of the filter elements will also increase. The filtering period may thus be automatically stopped and the backwash period initiated by monitoring of the rate of filtrate production, or by the automatic monitoring of the pressure differential between the inside and outside of the filter elements.
As established by the well-known art therefore, the process of filtration and backwashing is attained by reversing the pressure differential across the walls of the filter elements. Various means are used for attaining the requisite pressure differential. In most of the disclosed art the filtration period consists of pressure-feeding the vessel containing the filter elements with the mixture to be filtered and thickened. At this time the filtrate compartment is either vented to the atmosphere or, in designs where the filtrate compartment is enclosed, a vacuum may be applied to this compartment. In yet another design with an enclosed filtrate compartment, an amount of air forcibly compressed therein by the upflowing filtrate under the filtration feed pressure is subsequently used as a means of applying a pressure to the filtrate in the filtrate compartment for backwashing while closing off the mixture feed and releasing some of the thickened product from the bottom of the vessel, thus reversing the pressures and causing the reversed flow of liquid through the filter elements.
Other minor variations to the systems for reversing the differential pressures have been proposed. However, the common feature to all systems is that the mixture feed to the specific filter elements that are to be backwashed is stopped during the backwash cycle. In some disclosures, compressed air is fed into the filtrate compartment while releasing some of the thickened product from the bottom of the tank, in others the filtrate compartment remains open to the atmosphere while again releasing some of the thickened product from the vessel.
This invention relates to a new method of creating a reversal of differential pressures for backwashing purposes within a filter vessel, such as has been described. It is particularly applicable to the simple type of filter vessel where the filtrate compartment equipped with an overflow remains constantly open to the atmosphere for easy accessibility and maintenance of the filter elements, and the filtering pressure during the filtration period of the cycle is created only by line pressure of the mixture being fed into the vessel.